Use of boron or enriched boron 10 in UO2

ABSTRACT

The present invention provides a nuclear fuel assembly, where a boron-containing compound is used as a burnable poison and is distributed in a majority of the rods in the assembly. The assembly comprises a plurality of fuel rods, each fuel rod containing a plurality of nuclear fuel pellets, wherein at least one fuel pellet in more than 50% of the fuel rods in the fuel assembly comprises a sintered admixture of a metal oxide, metal carbide or metal nitride and a boron-containing compound.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part application based onU.S. Ser. No. 10/965,372, filed Oct. 14, 2004 now U.S. Pat. No.7,139,360, entitled Use of Boron or Enriched Boron 10 in UO₂.

FIELD OF THE INVENTION

The present invention relates to a nuclear fuel assembly, to be used ina nuclear power reactor. The fuel assembly contains fuel pellets havinga boron-containing compound in admixture with the nuclear fuel.

BACKGROUND INFORMATION

In a typical nuclear reactor, such as a pressurized water (PWR), heavywater or a boiling water reactor (BWR), the reactor core includes alarge number of fuel assemblies, each of which is composed of aplurality of elongated fuel elements or rods. The fuel rods each containfissile material such as uranium dioxide (UO₂) or plutonium dioxide(PuO₂), or mixtures of these, usually in the form of a stack of nuclearfuel pellets, although annular or particle forms of fuel are also used.The fuel rods are grouped together in an array which is organized toprovide a neutron flux in the core sufficient to support a high rate ofnuclear fission and thus the release of a large amount of energy in theform of heat. A coolant, such as water, is pumped through the core inorder to extract some of the heat generated in the core for theproduction of useful work. Fuel assemblies vary in size and designdepending on the desired size of the core and the size of the reactor.

When a new reactor starts, its core is often divided into a plurality,e.g. three or more groups of assemblies which can be distinguished bytheir position in the core and/or their enrichment level. For example, afirst batch or region may be enriched to an isotopic content of 2.0%uranium-235. A second batch or region may be enriched to 2.5%uranium-235, and a third batch may be enriched to 3.5% uranium-235.After about 10-24 months of operation, the reactor is typically shutdown and the first fuel batch is removed and replaced by a new batch,usually of a higher level of enrichment (up to a preferred maximum levelof enrichment). Subsequent cycles repeat this sequence at intervals inthe range of from about 8-24 months. Refueling as described above isrequired because the reactor can operate as a nuclear device only solong as it remains a critical mass. Thus, nuclear reactors are providedwith sufficient excess reactivity at the beginning of a fuel cycle toallow operation for a specified time period, usually between about sixto eighteen months.

Since a reactor operates only slightly supercritical, the excessreactivity supplied at the beginning of a cycle must be counteracted.Various methods to counteract the initial excess reactivity have beendevised, including insertion of control rods in the reactor core and theaddition of neutron absorbing elements to the fuel. Such neutronabsorbers, known in the art and referred to herein as “burnable poisons”or “burnable absorbers”, include, for example, boron, gadolinium,cadmium, samarium, erbium and europium compounds. Burnable poisonsabsorb the initial excess amount of neutrons while (in the best case)producing no new or additional neutrons or changing into new neutronpoisons as a result of neutron absorption. During the early stages ofoperation of such a fuel element, excess neutrons are absorbed by theburnable poison, which preferably undergoes transformation to elementsof low neutron cross section, which do not substantially affect thereactivity of the fuel element in the later period of its life when theneutron availability is lower.

Sintered pellets of nuclear fuel having an admixture of aboron-containing compound or other burnable poison are known. See, forexample, U.S. Pat. Nos. 3,349,152; 3,520,958; and 4,774,051. However,nuclear fuel pellets containing an admixture of a boron burnableabsorber with the fuel have not been used in large land-based reactorsdue to concerns that boron would react with the fuel, and because theuse of boron was thought to create high internal rod pressurization fromthe accumulation of helium in the reaction:¹⁰B+¹ n→ ¹¹B(excited state)→⁴He+⁷Li

Current practice is to coat the surface of the pellets with aboron-containing compound such as ZrB₂, which avoids any potentialreaction with the fuel. However, this does not solve the pressurizationproblem, which limits the amount of coating that can be contained withineach rod. More rods with a lower ¹⁰B loading must be used, thusnecessitating the handling and coating of a large number of fuelpellets, which is very expensive and results in high overhead costs.Complex manufacturing operations also result from the need to separatethe coated and non-coated fuel manufacturing and assembly operations. Inpractice, the cost of coating the pellets limits their use, and they areused in as few rods as possible, taking into account the pressurizationproblem described above. Historically this was acceptable, because fuelcycles were shorter, levels of ²³⁵U enrichment were lower, and overallthermal output of a reactor was lower.

Other compounds such as Gd₂O₃ and Er₂O₃ can be added directly to thepellets, but these are less preferred than boron because they leave along-lived, high cross-section residual reactive material.

Nuclear reactor core configurations having burnable poisons have beendescribed in the art. For example, U.S. Pat. No. 5,075,075 discloses anuclear reactor core having a first group of rods containing fissionablematerial and no burnable absorber and a second group of rods containingfissionable material with a burnable absorber, wherein the number ofrods in the first group is larger than the number of rods in the secondgroup. The burnable absorber comprises a combination of an erbiumcompound and a boron compound.

U.S. Pat. No. 5,337,337 discloses a fuel assembly where fuel rodscontaining a burnable poison element having a smaller neutron absorptioncross-section (such as boron) are placed in a region of the core havingsoft neutron energy and a large thermal neutron flux, while rods havinga burnable poison element having a larger neutron absorptioncross-section (such as gadolinium) are placed in regions of the corehaving average neutron energy spectrum. Neither of these prior patentsdisclose an arrangement of fuel rods in fuel assemblies in which amajority of fuel rods contain boron alone, as the burnable poison.Neither disclose assembly arrangements suitable for reactors producingover 500 megawatts thermal power.

With the use of longer fuel cycles and higher levels of ²³⁵U enrichment,there remains a need for the development of nuclear fuels and fuelassemblies having integral burnable absorbers that are cost-effectiveand can extend the life of the fuel without creating additional reactivematerials.

SUMMARY OF THE INVENTION

The present invention solves the above need by providing a fuel assemblycomprising a plurality of fuel rods, each fuel rod containing aplurality of nuclear fuel pellets, wherein at least one fuel pellet inmore than 50% of the fuel rods in the fuel assembly comprises a sinteredadmixture of an actinide oxide, actinide carbide or actinide nitride anda boron-containing compound. Due to the fact that boron has a relativelylow parasitic cross-section as compared to other burnable absorbers, itwill typically be necessary to put boron-containing fuel pellets in morethan 50% of the rods. It has been found, contrary to previousassumptions, that boron does not interact with the nuclear fuel, and isnot the primary cause of pressure in the fuel rods, when the amount ofhelium produced is compared to the amounts of other fission gasesreleased during fuel use. Preparing fuel with an admixture of boron ismuch less expensive. Therefore, a greater number of rods can have theboron-containing fuel pellets, providing a greater amount of boron inthe core but with less boron in each rod, thus avoiding thepressurization problem. For example, with the use of coated pellets fuelrods will contain about 2 mg boron per inch, whereas with the use ofboron directly in the pellet fuel rods will contain about 1-1.5 mg boronper inch, a 25-50% reduction.

By adding either natural or enriched boron to at least one fuel pelletin a majority of the rods in a fuel assembly, reactivity hold-down thatis equivalent or superior to that provided by current methods isprovided, at much lower cost. Additionally, increasing the number ofrods containing boron can reduce the internal fuel rod pressure by afactor of 2 or 3 over that found in current practice. Thus, using lowerlevels of a boron-containing compound, in combination with itsdistribution more widely among the fuel rods, provides the benefits ofthe present invention. As will be appreciated by one skilled in the art,these benefits are most advantageous when the thermal output of thereactor core is above 500 megawatts thermal, in the case of water-cooledreactors, or above 200 megawatts thermal in the case of gas-cooledreactors.

The use of boron in boiling water reactor fuel as a substitute for thecurrently employed Gd₂O₃ and Er₂O₃ provides even greater benefits. Inaddition to simplifying manufacturing, the space that is taken up by theGd₂O₃ and Er₂O₃ in the fuel pellets can be replaced by more UO₂ (orother actinide oxide, carbide or nitride), thus allowing more fuel to beloaded in a given size core. Enrichment constraints currently applied ona rod-by-rod basis due to poor thermal conductivity of these rare-earthoxides can be completely avoided, thus yielding a significantsimplification in the manufacture of nuclear fuels.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the following detailed description, reference will bemade to the attached drawings in which:

FIG. 1 is a longitudinal view, partly in section and partly inelevation, of a prior art nuclear reactor to which the present inventionmay be applied.

FIG. 2 is a simplified enlarged plan view of the reactor taken alongline 2-2 of FIG. 1, but with its core having a construction andarrangement of fuel and boron-containing compound in accordance with thepresent invention.

FIG. 3 is an elevational view, with parts sectioned and parts brokenaway for clarity, of one of the nuclear fuel assemblies in the reactorof FIG. 2, the fuel assembly being illustrated in verticallyforeshortened form.

FIG. 4 is enlarged foreshortened longitudinal axial sectional view of afuel rod of the fuel assembly of FIG. 3 containing a middle string ofboron-containing fuel pellets with upper and lower end strings ofuncoated fuel pellets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Accordingly, the present invention provides a fuel assembly comprising aplurality of fuel rods, each fuel rod containing a plurality of nuclearfuel pellets, wherein at least one fuel pellet in more than 50% of saidfuel rods in said fuel assembly comprises a sintered admixture of ametal oxide or metal nitride and a boron-containing compound. Theboron-containing compound functions as the burnable poison in the fuel.The term “fuel pellet” is used herein to denote the individual sinteredpellets of fuel that are loaded into a fuel rod. Preferably, at leastone fuel pellet in more than 60% of the fuel rods in the fuel assemblycontains a boron-containing compound. Even more preferably, at least onefuel pellet in more than 70-80% of the fuel rods in the fuel assemblycontains a boron-containing compound.

When referring to any numerical range of values herein, such ranges areunderstood to include each and every number and/or fraction between thestated range minimum and maximum. A range of more than 50% of the fuelrods in a fuel assembly, for example, would expressly include allintermediate values between 50 and 100%, including, by way of exampleonly, 51%, 52%, 53%, 60%, 70%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 100%,and all other intermediate values there between. In one embodiment, atleast one fuel pellet in more than 50% of the fuel rods in the fuelassembly comprises an admixture of a boron-containing compound and thenuclear fuel. In other embodiments, at least one fuel pellet in at least60%, 70%, 80%, 90% or more of the fuel rods in the fuel assembly containthe boron compound.

In the rods having at least one boron-containing fuel pellet, any numberof boron-containing fuel pellets can be used, to a maximum of 100% ofall the pellets in the rod. Typically, the number of fuel pelletscontaining boron in a rod will be greater than 50%, but the number ofboron-containing pellets in a particular rod will be determined based onall aspects of fuel design, as discussed further below.

Any suitable boron-containing compound can be used, so long as it iscompatible with the particular nuclear fuel selected and meets fuelspecifications as to density, thermal stability, physical stability, andthe like. Suitable boron-containing compounds include, but are notlimited to, ZrB₂, TiB₂, MoB₂, UB₂, UB₃, UB₄, B₂O₃, ThB₄, UB₁₂, B₄C,PuB₂, PuB₄, PuB₁₂, ThB₂, BN and combinations thereof. Preferredboron-containing compounds are BN, UB₁₂ and UB₄.

The boron-containing compound and actinide oxide, carbide or nitride areprepared as an admixture and then sintered to produce a fuel pellet.Such methods of preparing nuclear fuel pellets are known in the art; asdescribed above, see U.S. Pat. Nos. 3,349,152; 3,520,958; and 4,774,051.Natural boron or boron enriched in the ¹⁰B isotope can be used, and anylevel of enrichment of ¹⁰B above natural levels is suitable, dependingon certain factors. With the use of more enriched boron, the amount ofboron-containing compound needed overall decreases, allowing aconcomitant increase in fuel loading. However, enriched boron is moreexpensive than natural boron, and the amount of boron enrichment usedwill be a cost consideration balanced with other aspects of fuel design.

Accordingly, the amount of boron-containing compound present in a fuelpellet will range between about 5 ppm to about 5 wt %, more preferablybetween about 10 ppm and 20,000 ppm, based on the total amount offissile material in the fuel pellet, and the amount used will varydepending on the level of uranium enrichment, the level of boronenrichment, and other factors. One skilled in the art of fuel design caneasily determine the desired amount of boron-containing compound to usein a fuel pellet, and how many fuel pellets with this desired amount ofboron-containing compound to place in a particular number of rods in afuel assembly. Such calculations are routinely done in design of a fuelload, which must take into account the age of the fuel, the use patternand activity of the surrounding fuel, the level of uranium-235 in thefuel and the number of neutrons given off. By way of example only, theuse of an equal amount of natural boron in all the rods of a batch (ifneutronically acceptable) will require boron levels between about 66 and7,000 ppm, while the use of 100% enriched boron would reduce the levelof boron needed to between about 13 and 1200 ppm. It is recognized thatthe selective boration of individual rods might be preferableneutronically, similar to current poison distribution methods. Fuel rodshaving fuel pellets with natural boron only, enriched boron only, or acombination of pellets with natural and enriched boron, are allcontemplated as being embraced by the present invention.

The boron-containing compound can be used with any suitable nuclearfuel. Examples of suitable nuclear fuels include actinide oxides,actinide carbides and actinide nitrides. Exemplary fuels include, butare not limited to, UO₂, PuO₂, ThO₂, UN, (U, P)O₂, (U, P, Th)O₂, and (U,Th)O₂, other actinide oxides, actinide carbides and actinide nitrides,mixtures of actinide oxides, mixtures of actinide carbides, and mixturesof actinide nitrides.

The above described fuel assembly is suitable and economical for use infast breeder reactors, as well as reactors that are substantially basedon thermal fission such as light or heavy water nuclear reactors,including pressurized water reactors (PWR), boiling water reactors (BWR)and pressurized heavy water reactors (PHWR or CANDU). The fuel assemblyis also suitable for use in gas-cooled reactors. Preferrably, thethermal output of the reactor core of any of the above reactor typeswill be above 500 megawatts thermal in the case of water-cooledreactors, and above 200 megawatts thermal in the case of gas-cooledreactors.

In the following description, like reference numbers designate like orcorresponding parts throughout the several views. Also in the followingdescription, it is to be understood that such terms as “forward”,“rearward”, “left”, “right”, “upwardly”, “downwardly”, and the like, arewords of convenience and are not to be construed as limiting terms.

Referring now to the drawings, and particularly to FIGS. 1 and 2, thereis shown an embodiment of the present invention, by way of example onlyand one of many suitable reactor types, a pressurized water nuclearreactor (PWR), being generally designated by the numeral 10. The PWR 10includes a reactor pressure vessel 12 which houses a nuclear reactorcore 14 composed of a plurality of elongated fuel assemblies 16. Therelatively few fuel assemblies 16 shown in FIG. 1 is for purposes ofsimplicity only. In reality, as schematically illustrated in FIG. 2, thecore 14 is composed of a great number of fuel assemblies.

Spaced radially inwardly from the reactor vessel 12 is a generallycylindrical core barrel 18 and within the barrel 18 is a former andbaffle system, hereinafter called a baffle structure 20, which permitstransition from the cylindrical barrel 18 to a squared off periphery ofthe reactor core 14 formed by the plurality of fuel assemblies 16 beingarrayed therein. The baffle structure 20 surrounds the fuel assemblies16 of the reactor core 14. Typically, the baffle structure 20 is made ofplates 22 joined together by bolts (not shown). The reactor core 14 andthe baffle structure 20 are disposed between upper and lower core plates24, 26 which, in turn, are supported by the core barrel 18.

The upper end of the reactor pressure vessel 12 is hermetically sealedby a removable closure head 28 upon which are mounted a plurality ofcontrol rod drive mechanisms 30. Again, for simplicity, only a few ofthe many control rod drive mechanisms 30 are shown. Each drive mechanism30 selectively positions a rod cluster control mechanism 32 above andwithin some of the fuel assemblies 16.

A nuclear fission process carried out in the fuel assemblies 16 of thereactor core 14 produces heat which is removed during operation of thePWR 10 by circulating a coolant fluid, such as light water with solubleboron, through the core 14. More specifically, the coolant fluid istypically pumped into the reactor pressure vessel 12 through a pluralityof inlet nozzles 34 (only one of which is shown in FIG. 1). The coolantfluid passes downward through an annular region 36 defined between thereactor vessel 12 and core barrel 18 (and a thermal shield 38 on thecore barrel) until it reaches the bottom of the reactor vessel 12 whereit turns 180 degrees prior to following up through the lower core plate26 and then up through the reactor core 14. On flowing upwardly throughthe fuel assemblies 16 of the reactor core 14, the coolant fluid isheated to reactor operating temperatures by the transfer of heat energyfrom the fuel assemblies 16 to the fluid. The hot coolant fluid thenexits the reactor vessel 12 through a plurality of outlet nozzles 40(only one being shown in FIG. 1) extending through the core barrel 18.Thus, heat energy which the fuel assemblies 16 impart to the coolantfluid is carried off by the fluid from the pressure vessel 12.

Due to the existence of holes (not shown) in the core barrel 18, coolantfluid is also present between the barrel 18 and the baffle structure 20and at a higher pressure than within the core 14. However, the bafflestructure 20 together with the core barrel 18 do separate the coolantfluid from the fuel assemblies 16 as the fluid flows downwardly throughthe annular region 36 between the reactor vessel 12 and core barrel 18.

As briefly mentioned above, the reactor core 14 is composed of a largenumber of elongated fuel assemblies 16. Turning to FIG. 3, each fuelassembly 16, being of the type used in the PWR 10, basically includes alower end structure or bottom nozzle 42 which supports the assembly onthe lower core plate 26 and a number of longitudinally extending guidetubes or thimbles 44 which project upwardly from the bottom nozzle 42.The assembly 16 further includes a plurality of transverse support grids46 axially spaced along the lengths of the guide thimbles 44 andattached thereto. The grids 46 transversely space and support aplurality of fuel rods 48 in an organized array thereof. Also, theassembly 16 has an instrumentation tube 50 located in the center thereofand an upper end structure or top nozzle 52 attached to the upper endsof the guide thimbles 44. With such an arrangement of parts, the fuelassembly 16 forms a integral unit capable of being conveniently handledwithout damaging the assembly parts.

As seen in FIGS. 3 and 4, each of the fuel rods 48 of the fuel assembly16 has an identical construction insofar as each includes an elongatedhollow cladding tube 54 with a top end plug 56 and a bottom end plug 58attached to and sealing opposite ends of the tube 54 defining a sealedchamber 60 therein. A plurality of nuclear fuel pellets 62 are placed inan end-to-end abutting arrangement or stack within the chamber 60 andbiased against the bottom end plug 58 by the action of a spring 64placed in the chamber 60 between the top of the pellet stack and the topend plug 56.

In the operation of a PWR, it is desirable to prolong the life of thereactor core 14 as long as feasible to better utilize the uranium fueland thereby reduce fuel costs. To attain this objective, it is commonpractice to provide an excess of reactivity initially in the reactorcore 14 and, at the same time, provide means to maintain the reactivityrelatively constant over its lifetime.

FIGS. 2, 3 and 4 illustrate a preferred embodiment of the presentinvention, to achieve this objective. As can be seen in FIGS. 3 and 4, afuel rod 48 has some end-to-end arrangements, or strings, of fuelpellets 62A containing no boron compound, provided at upper and lowerend sections of the fuel pellet stack of the fuel rod 48 as an axialblanket. The fuel rod 48 also has a string of the fuel pellets 62B withthe boron-containing compound provided at the middle section of thestack.

Referring to FIG. 2, there is shown one preferred embodiment of anarrangement in the nuclear reactor core 14 in accordance with thepresent invention, of assemblies with fuel rods having noboron-containing compound, denoted by an “o” in FIG. 2, and assembliesin which all the fuel rods in the assembly have at least one pellet offuel with a boron-containing compound, denoted by an “x” in FIG. 2.

By way of example only, Table 1 below provides information comparing anassembly of the present invention with prior art practice.

TABLE 1 Original Rods Rods with With IFBA-coated UB₄ Fuel (ZrB₂)(present invention) Boron loading 10 mg/inch 325.5 ppm Percent of allrods coated 60% 100% With ZrB₂ or containing UB₄ Pellet diameter 0.37inches 0.37 inches UO₂ density 10.47 gm/cm³ 10.47 gm/cm³ UO₂ loading18.43 gm UO₂/inch 18.43 gm UO₂/inch ¹⁰B loading 108.5 ppm 65.1 ppm ¹⁰Blevel in total amount 20% 20% of Boron Smeared ¹⁰B loading 65.1 ppm 65.1ppm Total B loading 524.5 ppm 325.5 ppm UB₄ loading 2119 ppm UB₄ % ofpellets with IFBA or 100% 100% UB₄

The invention provides that any suitable boron-containing compound canbe used as long as it is compatible with the particular nuclear fuelselected and further meets fuel specifications as to density, thermalstability, physical stability and the like. In a preferred embodiment,the suitable boron-containing compound is boron nitride (BN). Naturalboron or boron enriched with a ¹⁰B isotope can be used in thisembodiment. Any ratio of non-enriched-to-enriched boron can be used withthe understanding that with the increased use of enriched boron, lessboron nitride needs to be added to the fuel pellet.

In the preferred embodiment, boron nitride is prepared as an admixturewith an actinide nitride nuclear fuel, and then sintered to produce afuel pellet. Preferred actinide nitrides include uranium nitride (UN),plutonium nitride (PN) and thorium nitride (ThN) or any combinationthereof.

By admixturing boron nitride with one or any combination of uraniumnitride, plutonium nitride or thorium nitride, a unity of nitridecompounds is achieved throughout the admixture. The unity of nitridecompounds in the admixture gives the boron-containing compound of theinvention improved compatibility with the actinide compound of theinvention. Further, the combination of boron nitride with one or anycombination of uranium nitride, plutonium nitride or thorium nitridedoes not significantly affect the properties of the actinide matrix.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A fuel assembly comprising a plurality of fuel rods, each fuel rodcontaining a plurality of nuclear fuel pellets, wherein at least onefuel pellet in more than 50% of said fuel rods in said fuel assemblycomprises a sintered admixture of an actinide compound and aboron-containing compound, wherein the boron-containing compound isboron nitride, and wherein the actinide compound is constituted of oneor a multiplicity of actinides compounded with nitrogen, the actinideselected from the group consisting of uranium, plutonium, thorium and acombination thereof.
 2. The fuel assembly of claim 1, wherein saidboron-containing compound in said at least one fuel pellet comprisesenriched boron.
 3. The fuel assembly of claim 2, wherein said enrichedboron is enriched to a content of ¹⁰B greater than natural boron.
 4. Thefuel assembly of claim 1, wherein said boron-containing compound ispresent in said fuel pellet in an amount of about 5 ppm to about 5 wt %,based on a total amount of fuel in said fuel pellet.
 5. The fuelassembly of claim 4, wherein said boron-containing compound is presentin said fuel pellet in an amount of about 10 ppm to about 20,000 ppm,based on the total amount of fuel in said fuel pellet.
 6. The fuelassembly of claim 5, wherein said boron-containing compound is presentin said fuel pellet in an amount of about 600 ppm to about 20,000 ppm,based on the total amount of fuel in said fuel pellet.
 7. The fuelassembly of claim 1, wherein at least one fuel pellet in at least 60% ofsaid fuel rods in said fuel assembly comprises a sintered admixture ofan actinide compound and a boron-containing compound.
 8. The fuelassembly of claim 1, wherein at least one fuel pellet in at least 80% ofsaid fuel rods in said fuel assembly comprises a sintered admixture ofan actinide compound and a boron-containing compound.
 9. The fuelassembly of claim 1, wherein at least one fuel pellet in at least 50%and less than 100% of said fuel rods in said fuel assembly comprises asintered admixture of an actinide compound and a boron-containingcompound.
 10. A light water reactor having a fuel assembly, the fuelassembly comprising a plurality of fuel rods, each fuel rod containing aplurality of nuclear fuel pellets not requiring coatings, wherein atleast one fuel pellet in more than 50% of said fuel rods in said fuelassembly comprises a sintered admixture of an actinide compound and aboron-containing compound, wherein the boron-containing compound isboron nitride, and wherein the actinide compound is constituted of oneor a multiplicity of actinides compounded with nitrogen, the actinideselected from the group consisting of uranium, plutonium, thorium, and acombination thereof.
 11. The light water reactor of claim 10, whereinsaid boron-containing compound in said at least one fuel pelletcomprises enriched boron.
 12. The light water reactor of claim 11,wherein said enriched boron is enriched to a content of ¹⁰B greater thannatural boron.
 13. The light water reactor of claim 10, wherein saidboron-containing compound is present in said fuel pellet in an amount ofabout 5 ppm to about 5 wt percent, based on a total amount of fuel insaid fuel pellet.
 14. The light water reactor of claim 13, wherein saidboron-containing compound is present in said fuel pellet in an amount ofabout 10 ppm to about 20,000 ppm, based on the total amount of fuel insaid fuel pellet.
 15. The light water reactor of claim 14, wherein saidboron-containing compound is present in said fuel pellet in an amount ofabout 600 ppm to about 20,000 ppm, based on the total amount of fuel insaid fuel pellet.
 16. The light water reactor of claim 10, wherein atleast one fuel pellet in at least 60% of said fuel rods in said fuelassembly comprises a sintered admixture of an actinide compound and aboron-containing compound.
 17. The light water reactor of claim 10,wherein at least one fuel pellet in at least 80% of said fuel rods insaid fuel assembly comprises a sintered admixture of an actinidecompound and a boron-containing compound.
 18. The light water reactor ofclaim 10, wherein at least one fuel pellet in at least 50% and less than100% of said fuel rods in said fuel assembly comprises a sinteredadmixture of an actinide compound and a boron-containing compound.